Joints of animal and human bodies are miraculous mechanical devices. They often last a lifetime with no added lubrication or service. However, the joints, like any mechanical or biological structure, are subject to certain failures. The joints are naturally lubricated and cushioned by synovial membranes and cartilages so that they normally appear subject to little wear. Part of this apparent lack of wear is doubtless due to living tissue's ability of regeneration and self-repair. However, joints are not invincible. Sometimes the body's immune system goes awry and attacks a joint, thereby damaging it irreversibly (rheumatoid arthritis). Sometimes old age and general wear and tear catches up with the joint's biological repair system (osteoarthritis). Sometimes a sharp blow or overextension of the joint results in mechanical damage that cannot be repaired by the normal healing process.
The knee joint in animals and humans is a frequent place for joint damage. The knee is a common source of problems because the joint has an unusually large range of motion and bears the weight of the entire body.
This wide range of motion requires extensive contact surface between the femur and the tibia. The joint is rather loosely held together by tendons and ligaments to permit such a wide range of motion. A front-facing side of the knee joint is protected by a separate knee cap (patella) which is held in place by ligaments and slides on a femoral joint surface (trochlear groove) as the knee bends. The patella and its ligaments are mechanically involved in joint extension. If any of the joint surfaces (femoral surface, patellar surface, or tibial surface) becomes damaged or roughened, the knee joint will not operate properly. Damage to the knee can cause pain and/or affect the ability to walk. Changes in operation of the knee from damage can cause further deterioration of the joint. Furthermore, changes in gait caused by adjustments to the damaged knee can result in additional stresses on and damage to other joints.
A common problem is damage to the patello-femoral joint so that free motion of the patella is inhibited and painful. This joint is particularly susceptible to repetitive damage. Dislocation of the knee cap can cause damage or unexpected wear to the edges of the trochlear groove. This makes the subsequent dislocations more probable. Additionally, in some people and animals, their gait may cause uneven wear on the edges of the trochlear groove so that one side becomes more worn. Thus, the knee cap fails to remain in position.
Such damage is illustrated in FIGS. 1 and 2a-2c. For purposes of illustration, the failure of the trochlear groove is illustrated in connection with the knee of a dog. Although the specific geometry of the knee joints in humans and other animals varies, the main components and relationships are the same across all knee joints. Therefore, the problems, and the solutions provided in the present invention, are equally applicable to prosthesis of humans and different animals. FIG. 1 illustrates a typical knee joint 1. Only the elements of the joint relevant to its operation in connection with the present invention are shown. The knee joint 1 includes the femur 2, tibia 3 and patella 4. The patella 4 is positioned in front of the condyles of the femur 2 and glides within the trochlear groove 6. Patellar ligament 5 is connected from the tibia 3 to the quadriceps tendon 7 across the patella 4 to keep the patella 4 in place.
FIGS. 2a-2c illustrate a cross section of the knee joint along the A-A plane in FIG. 1. FIG. 2a illustrates a normal femur 2 having a pronounced trochlear groove 6. The medial side 21 of the femur has an extension 23 on the medial side of the trochlear groove 6. Similarly, an extension 24 of the lateral side 22 of the femur is positioned on the lateral side of the trochlear groove 6. The trochlear groove 6 itself has a unique shape to retain the patella 4 in position and to allow for proper movement of the patella 4 on the femur 2. Damage may occur to the extensions 23, 24 of the medial 21 and lateral 22 sides of the femur, as illustrated in FIGS. 2b and 2c respectively. If medial side extension 23′ is damaged or excessively worn (FIG. 2b), the patella 4 can slide 4′ off the medial side 21 of the femur. In fact, operation of forces on the knee will likely cause such motion and may cause further damage to the medial side extension 23′. Similarly, excessive wear or damage to the lateral side extension 24′ (FIG. 2c) will cause the patella 4 to slide 4″ off the lateral side 22 of the femur.
Such damage can make normal joint movement almost impossible. At one time, before the mechanical and protective functions of the patella were understood, the patella was simply removed in an attempt to cure patello-femoral problems. Today, a variety of prosthetic replacements have been developed for different joint surfaces of the knee joint. In extreme cases the entire joint can be replaced with a prosthetic device. However, such surgery naturally requires a considerable time for recovery. In less extreme cases it may be advantageous to replace only the damaged part of the joint. Generally, correction of the patello-femoral joint requires extensive replacement of portions of the femur, including the portions contacting the tibia. Furthermore, such prosthesis merely provide a through to limit lateral movement of the patella. They fail to replicate the unique shapes of the trochlear groove so that correct operation of the joint is achieved.
U.S. Pat. No. 3,806,961 to Miller shows a prosthesis having mating femur and patella parts. An annular sector having a guide groove is implanted into the end of the femur. A raised arcuate runner member is implanted into the patella so that the patella can slidably move in the guide groove with the runner member acting as a bearing surface. The prostheses are attached to the bone surfaces by bone cement.
U.S. Pat. No. 3,878,566 to Bechtol discloses another patellar prosthesis. Here a femoral implant bears a more or less acute groove, and the patellar component bears a somewhat crest-like ridge projection that rides in the groove. U.S. Pat. No. 4,007,495 to Frazier uses a slightly different approach. The patellar component rides in a femoral groove, but the system is also equipped with a femoral projection that engages a slot in the patellar implant. While this structure prevents separation of the patella from the femur, it also greatly restricts movement of the patella and may result in unnatural joint action.
U.S. Pat. No. 4,151,615 to Hall provides a femoral component and a patellar component that more closely approximate natural patello-femoral joint motion.
U.S. Pat. No. 4,838,891 to Branemark et al. addresses the adhesion problems of prior prostheses by providing a two-part system where an anchoring device is inserted during a first operation. After the healing process fixes the anchoring device firmly in place, a second operation inserts the weight-bearing part of the prosthesis which engages the anchoring device. However, this system requires multiple operations even to insert a simple patello-femoral prosthesis.
U.S. Pat. No. 5,824,098 to Stein replaces the femoral surface of the patella with a convex prosthesis of a low friction material. The trochlear groove is replaced with an elongated femoral prosthesis having an arcuate indentation on its upper surface to receive the patella prosthesis.
All of these prior art prostheses require extensive surgery to access all parts of the patella and femur. They require extensive removal of portions of the femur, which may include sound material. They cause alteration of the operation of the patello-femoral joint. Therefore, a need exists for a simple prosthesis for replacing only the trochlear groove with minimally invasive surgery.